The number of fetuses can usually be determined by ultrasound examination at around 40 days of pregnancy. At around 40 days of pregnancy, the head, frontal organs, respiratory, digestive and nervous organs of the fetus have gradually begun to differentiate, and the number of fetuses can be initially determined through ultrasound examination as the fetal sac can be clearly seen, as well as the fetal buds and the fetal heart. If the result shows 2 fetal hearts and 1 gestational sac, this is monozygotic twins, usually 2 girls or 2 boys, because their genes are the same, so the fetuses will look very similar. If the result shows 2 fetal hearts and 2 gestational sacs, it means dizygotic twins, which may be dragon and phoenix twins, even if they are 2 girls or 2 boys, they will look different due to their different genes. If the result shows only 1 fetal heart and 1 gestational sac, it is a singleton. At this time, the embryo is in the differentiation stage, and is very vulnerable to external factors. Pregnant women should pay attention to supplementing folic acid, which can effectively prevent the fetus from developing neural tube malformations and other pathologies. On time delivery checkups, pay attention to stay away from radiation, chemicals and other toxic, harmful environment, to avoid affecting the growth and development of the fetus. Pregnant women should also pay attention to maintain a relaxed and happy mood, pay attention to personal hygiene in daily life, avoid infection, and reduce the opportunity to use medication.